1. Field of the Invention
The present invention relates to a method of plasma etching and a method of operating a plasma etching apparatus.
2. Discussion of the Background
In the fabrication of semiconductor devices, numerous conductive device regions and layer are formed in or on a semiconductor substrate. The conductive regions and layers of the device are isolated from one another by a dielectric, for example, silicon dioxide. The silicon dioxide may be grown, or may be deposited by physical deposition (e.g., sputtering) or by a variety of chemical deposition methods and chemistries. Additionally, the silicon dioxide may be undoped or may be doped, for example, with boron, phosphorus, or both, to form for example, borophosphosilicate glass (BPSG), and phosphosilicate glass (PSG). The method of forming the silicon dioxide layer and the doping of the silicon dioxide will depend upon various device and processing considerations. Herein, all such silicon dioxide layers are referred to generally as "oxide" layers.
At several stages during fabrication, it is necessary to make openings in the dielectric to allow for contact to underlying regions or layers. Generally, an opening through a dielectric layer between polysilicon and the first metal layer is called a "contact opening", while an opening in other oxide layers such as an opening through an intermetal dielectric layer (ILD) is referred to as a "via". As used herein, an "opening" will be understood to refer to any type of opening through any type of oxide layer, regardless of the stage of processing, layer exposed, or function of the opening.
During semiconductor manufacturing, it is typically necessary to conduct selective etching of material such as in the formation of contacts. A common technique for etching overlaying dielectric layers is photolithography, in which light is used to form a pattern on a photosensitive film which has been deposited on the surface of a dielectric layer. Development of the resist results in a pattern, in which portions of the oxide are exposed. The exposed portions of the oxide may then be subject to selective etching to form a contact.
Etching may be conducted by many methods, however, plasma based processes such as plasma enhanced chemical vapor deposition and reactive ion etching (RIE) are very common. Typically, the plasma is generated by coupling radio frequency (RF) electromagnetic energy to the plasma. The RF energy is supplied by an RF generator coupled to a power supply. Because the plasma has a variable impedance, a matching network is employed to match the impedance of the power supply with that of the plasma. The matching network may include one or more capacitors and one or more inductors to achieve the match and thereby tune the RF power. Typically, the tuning may be done automatically by an automatic matching network (AMN). When tuned, most of the power output of the RF generator is coupled to the plasma. The power to the plasma is often referred to as forward power.
However, etching of wafers to form contact openings is sometimes complicated by the redeposition of species which have a volatility sufficiently low to lead to localized micromasking effects called grass. The redeposited species act as a local mask while the surrounding area is being etched away by surface gasification. By the time the localized masks get removed the surface area is very rough and appears to have grass-like features sticking up from the bottom. Processing efforts designed to consistently eliminate or minimize the formation of such seal ring residues and grass have been met with little success.
Many of the etch characteristics are generally believed to be affected by polymer residues which deposit during the etch. For this reason, the fluorine to carbon ratio (F/C) in the plasma is considered an important determinant in the etch. In general, a plasma with a high F/C ratio will have a faster etch rate than a plasma with a low F/C ratio. At very low F/C ratios (i.e., high carbon content), polymer deposition may occur and etching may be reduced. The etch rate as a function of the F/C ratio is typically different for different materials. This difference is used to create a selective etch, by attempting to use a gas mixture which puts the F/C ratio in the plasma at a value that leads to etching at a reasonable rate for one material, and that leads to little or no etching or polymer deposition for another.
In addition, excessive net polymer deposits sometimes occurs while the Rf component of the etch system is stabilizing at the target value. For a more thorough discussion of oxide etching, see S. Wolf and R. N. Tauber, Silicon Processing for the VLSI ERA, Volume 1, pp 539-585 (1986).
The introduction of oxygen into an etching process has been reported to allow for control of the anisotropy, by varying the fraction of O.sub.2 in the feed (see for example Burton et al. J. Electrochem. Soc.: Solid-State Science and Technology, v 129, no 7, 1599 (1982)).
Accordingly, a plasma etching method which provides for reliable etching of a dielectric layer which does not suffer from a residue formation problem is sought.
The inventors of the present invention have discovered that grass formation can be reduced and nearly eliminated by the addition of O.sub.2 during the striking of the plasma.